We are studying some effects of the major environmental carcinogen, ultraviolet (UV) radiation, on human and other eukaryotic cells. Three unprecedented observations were made in this laboratory about the effect of UV radiation on the accumulation of newly synthesized small nuclear RNA (snRNA) species U1, U2, U3, U4, and U5: (1) although U snRNAs are small (for example, human U1 RNA is 165 nucleotides long), their biosynthesis is very sensitive to UV radiation; (2) the inactivation curve of their synthesis is monophasic, instead of the usual single slope; and (3) the inhibition of U1-U5 snRNA synthesis increases within the first two hours after UV irradiation (although inhibited transcription usually returns to normal as pyrimidine dimers are removed from DNA). We now wish to investigate the mechanisms behind these puzzling observations. This includes determining the step within the U snRNA biosynthesis pathway which is affected by UV radiation, e.g., initiation, elongation, or termination of transcription?). Another goal is to find out what the critical alteration involved in this inhibition is; e.g., formation of pyrimidine dimers, hypersensitivity to postirradiation repair nicks in DNA, or ensuing changes in DNA torsional stress. An additional objective is to define the portion of genomic DNA required to obtain the inhibitory effects of UV radiation on U1-U5 snRNA synthesis. The approaches to be used are: radiolabeling experiments with whole cells including hybrid selection assays; transcription in isolated nuclei and in a soluble in vitro system; transcription in thymidine kinase-deficient cells that have been transfected with cloned U1 genes. (H)